Chlorination/Dechlorination: A series of maze styled tanks is where the clarified water from the secondary clarifiers flows by gravity. Here chlorine is added before entering the chlorine contact tanks. There has to be a minimum of 30-minutes of contact time with this clarified secondary effluent. The chlorine kills most of the harmful bacteria. Hypochlorite is the form of chlorine used to reduce bacteria and pathogenic organisms.
Chlorine can harm the environment by mixing with ammonia to form chloroamines that are harmful to fish and other aquatic organisms. Because of this, the chlorinated effluent must be neutralized to offset the lethal effects of the chlorine on fish and other organisms. Sodium bisulfite is added at the end of the mazed tanks where a minimum of a two-minute reaction time is needed to assure the chlorine is bound and neutralized into harmless salts (chloride salts).
This dechlorinated final effluent is what flows over the weirs of the tanks, across a channel then through the outfall discharge to the Merrimack River. The average daily discharge of treated effluent is approximately 25 million gallons of cleaned wastewater.